Trench-based power semiconductor devices with increased breakdown voltage characteristics

ABSTRACT

Exemplary power semiconductor devices with features providing increased breakdown voltage and other benefits are disclosed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/667,319, filed Nov. 2, 2012, which is a continuation of U.S. patentapplication Ser. No. 12/408,552, filed Mar. 20, 2009 (now U.S. Pat. No.8,304,829), which claims the benefit of U.S. provisional patentapplication No. 61/120,818, filed Dec. 8, 2008, all of which areincorporated herein by reference in their entireties for all purposes.

BACKGROUND

Exemplary power semiconductor devices include planar-gate MOSFETtransistors, vertical gate MOSFET transistors, insulated-gate bipolartransistors (IGBTs), rectifiers, and synchronous rectifiers. Typicalimplementations of the trench-gate variety of these devices comprise anarray of trenches formed in the top surface of the semiconductor die,with each trench filled with a shield electrode and/or a gate electrode,depending upon the type of power device. The trenches define acorresponding array of mesas, each mesa being disposed between adjacenttrenches. Depending upon the device implemented on the die, variouselectrodes and/or doped regions are disposed at the top of the mesa.Each mesa and its adjacent trenches implement a small instance of thedevice, and the small instances are coupled together in parallel toprovide the whole power semiconductor device. The whole device has an ONstate where a desired current flows through the device, an OFF statewhere current flow is substantially blocked in the device, and abreakdown state where an undesired current flows due to an excessoff-state voltage being applied between the current conductingelectrodes of the device. The voltage at which breakdown is initiated iscalled the breakdown voltage. Each mesa and its adjacent trenches areconfigured to provide a desired set of ON-state characteristics andbreakdown voltage. There are various tradeoffs in the design of the mesaand trenches between achieving good ON-state characteristics, highbreakdown voltage, and improved switching characteristics.

A typical power semiconductor die has an active area where the array ofmesas and trenches that implement the device are located, a fieldtermination area around the active area, and an inactive area whereinterconnects and channel stops may be provided. The field terminationarea minimizes the electric fields around the active area, and is notintended to conduct current. Ideally, one would like the device'sbreakdown voltage to be determined by the breakdown processes associatedwith the active area. However, there are various breakdown processesthat can occur in the field termination area and inactive area atsignificantly lower voltages. These breakdown processes may be referredto as passive breakdown processes.

Much effort has been made in the prior art to design field terminationareas that have higher breakdown voltages than the active area. However,such prior art designs often fall short of this goal, often requiringcompromises that increase the total die area and cost of the die.

BRIEF SUMMARY

The inventors have discovered several locations in trench-based powerdevices where parasitic breakdown conditions are likely to occur first.The present application provides novel and inventive features thatcounter these breakdown conditions and increase breakdown voltage.

Aspects of the exemplary embodiments of the present invention describedherein may be used alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of an exemplary semiconductor die thatincorporates several features according to the present invention.

FIG. 2 shows a magnified view of the left top corner of the exemplarysemiconductor die of FIG. 1 according to the present invention.

FIG. 3 shows a magnified view of a portion of the left side of theexemplary semiconductor die of FIG. 1 according to the presentinvention.

FIGS. 4 and 5 show a first cross section view of a portion of theexemplary semiconductor die of FIG. 1 and a magnified view thereof inFIG. 5 according to the present invention.

FIG. 6 shows a magnified cross section view of a portion of a variationof the exemplary semiconductor die of FIG. 1 according to the presentinvention.

FIGS. 7-14 show various magnified cross section views of the exemplarysemiconductor die of FIG. 1 and possible variations thereof according tothe present invention.

FIG. 15 shows a top view of another exemplary semiconductor die thatincorporates several features according to the present invention.

FIGS. 16-19 show various magnified cross section views of the exemplarysemiconductor die of FIG. 15 and possible variations thereof accordingto the present invention.

FIG. 20 shows a top view of another exemplary semiconductor die thatincorporates several features according to the present invention.

FIGS. 21-29 show various magnified cross section views of the exemplarysemiconductor die of FIG. 20 and possible variations thereof accordingto the present invention.

FIG. 30 shows a top view of another exemplary semiconductor die thatincorporates several features according to the present invention.

FIG. 31 show a magnified cross section view of the exemplarysemiconductor die of FIG. 30 according to the present invention.

FIG. 32 shows a top view of another exemplary semiconductor die thatincorporates several features according to the present invention.

FIGS. 33-36 show various magnified cross section views of the exemplarysemiconductor die of FIG. 32 and possible variations thereof accordingto the present invention.

FIGS. 37-39 show various magnified cross section views of anotherexemplary semiconductor and possible variations thereof thatincorporates several features according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The techniques in accordance with the present inventions will bedescribed more fully hereinafter with reference to the accompanyingdrawings, in which exemplary embodiments of the invention are shown.This invention may, however, be embodied in different forms and shouldnot be construed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure is thorough andcomplete and fully conveys the scope of the invention to one skilled inthe art. In the drawings, the thicknesses of layers and regions may beexaggerated for clarity. The same reference numerals are used to denotethe same elements throughout the specification. The elements may havedifferent interrelationships and different positions for differentembodiments.

It will also be understood that when a layer is referred to as being“on” another layer or substrate, it can be directly on the other layeror substrate, or intervening layers may also be present. It will also beunderstood that when an element, such as a layer, a region, or asubstrate, is referred to as being “on,” “connected to,” “electricallyconnected to,” “coupled to,” or “electrically coupled to” anotherelement, it may be directly on, connected or coupled to the otherelement, or one or more intervening elements may be present. Incontrast, when an element is referred to as being “directly on,”“directly connected to” or “directly coupled to” another element orlayer, there are no intervening elements or layers present. It may beappreciated that the claims of the application may be amended to reciteexemplary relationships described in the specification or shown in thefigures with the support thereof being provided by the originalapplication. The term “and/or” used herein includes any and allcombinations of one or more of the associated listed items.

The terms used herein are for illustrative purposes of the presentinvention only and should not be construed to limit the meaning or thescope of the present invention. As used in this specification, asingular form may, unless definitely indicating a particular case interms of the context, include a plural form. Also, the expressions“comprise” and/or “comprising” used in this specification neither definethe mentioned shapes, numbers, steps, actions, operations, members,elements, and/or groups of these, nor exclude the presence or additionof one or more other different shapes, numbers, steps, operations,members, elements, and/or groups of these, or addition of these.Spatially relative terms, such as “over,” “above,” “upper,” “under,”“beneath,” “below,” “lower,” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. It willbe understood that the spatially relative terms are intended toencompass different orientations of the device in use or operation inaddition to the orientation depicted in the figures. For example, if thedevice in the figures is turned over, elements described as “below” or“beneath” or “under” other elements or features would then be oriented“over” or “above” the other elements or features. Thus, the exemplaryterm “above” may encompass both an above and below orientation.

As used herein, terms such as “first,” “second,” etc. are used todescribe various members, components, regions, layers, and/or portions.However, it is obvious that the members, components, regions, layers,and/or portions should not be defined by these terms. The terms are usedonly for distinguishing one member, component, region, layer, or portionfrom another member, component, region, layer, or portion. Thus, a firstmember, component, region, layer, or portion which will be described mayalso refer to a second member, component, region, layer, or portion,without departing from the scope of the present invention.

FIG. 1 shows a top view of an exemplary semiconductor device 100 thatincorporates several features according to the present invention. Device100 comprises an active device region 120 located in the middle of thedie. Without loss of generality, device region 120 may implement avertical, trench-shielded power MOSFET device. As described and shownbelow in greater detail, the exemplary MOSFET device comprises an arrayof trenches interleaved with an array of mesas, insulated shieldelectrodes disposed in bottoms of the trenches, insulated gateelectrodes disposed in the trenches over the shield electrodes, sourceregions disposed in the mesas, source electrodes disposed on the sourceregions, and a drain electrode provided at the backside of thesemiconductor device. Device 100 further comprises a source metal layer110 (also called conductive layer 110) disposed over device region 120and electrically coupled to the source electrodes, and a source pad 111disposed over conductive layer 110 and electrically coupled thereto, andin turn to the source regions of the power MOSFET device. Source pad 111is adapted to receive an external connection, such as a wire bond orsolder bump that provides a source potential, and may have dimensions of150 microns on each side.

On each of the left and right sides of the device region 120, device 100further comprises a connection region 150 where electrical contact ismade to the gate and shield electrodes that are disposed in thetrenches. In each connection region, a stripe of conductive material,called a gate runner, is disposed parallel to a side of device region120 and spaced therefrom. The gate runner makes electrical contact withthe gate electrodes in the trenches, but is electrically isolated fromthe mesas that are interleaved between the trenches. Each gate runner iselectrically coupled to a gate pad 112 located at the bottom of the die.The gate pad 112 is adapted to receive an external connection, such as awire bond or solder bump that provides a gate potential. Also in eachconnection region 150, another stripe of conductive material, called ashield runner, is disposed parallel to the gate runner and spacedtherefrom. The shield runner makes electrical contact with the shieldelectrodes in the trenches, but is electrically isolated from portionsof the mesas that it overlies. The shield runners are electricallycoupled to the source conductive layer by an extension of the sourceconductive layer at the top of the die, or to a shield Pad and using anexternal connection.

A channel stopper is disposed at or near the periphery of the die, andis spaced from the shield runners and the top portion of device region120 by a gap. The channel stopper is conventional, and may comprise anisolated ring of metal that overlays and makes contact to a strip ofdoped semiconductor region that forms a ring around the periphery of thedie. Of significant note, die 100 does not comprise the conventionalfield termination structures that would normally appear in this gap.

FIG. 2 shows a magnified view of the upper left-hand corner of die 100,and FIG. 3 shows a magnified view of a portion along the left side ofthe die. The above features may be more clearly seen in these figures.FIGS. 2 and 3 provide reference points for a number of cross sections ofdie 100 that will be discussed next.

FIG. 4 is a cross section view of a portion of the die 100 that includesactive device region 120 and a first field termination region. Die 100comprises a N+ doped semiconductor substrate 102, one or moreepitaxially grown semiconductor n-type layers 104 (“epitaxialsemiconductor layer”) disposed on semiconductor substrate 102, an oxidelayer 106 disposed over epitaxial semiconductor layer 104 in theinactive and first field termination regions, a dielectric layer 107disposed over the oxide layer 106, a gate runner disposed over thedielectric layer 107 at the left portion of the inactive region, andconductive layer 110 (source metal layer 110) disposed over dielectriclayer 107 in the first field termination region. As is known in the art,a semiconductor region may be doped as a p-conductivity type (or“p-type”) region with a p-type dopant, or doped as an n-conductivitytype (or “n-type”) region with an n-type dopant. In device region 120,device 100 further comprises a plurality of trenches 122 disposed in theepitaxial semiconductor layer, and a plurality of mesas 130 ofsemiconductor material interleaved between trenches 122. Portions of thedielectric layer 107 cover the tops of trenches 122, and the sourcemetal layer 110 extends over active device region 120 and makes contactto mesas 130. The structure of trenches 122 and mesas 130 is describedbelow with reference to FIG. 5. In the first termination region, device100 further comprises a first end trench 222, a first end mesa 230disposed between first end trench 222 and the leftmost trench 122 ofdevice region 120, and a second end mesa 238 disposed to the left offirst end trench 222.

FIG. 5 is a magnified cross section view of the first field terminationregion and device region 120 shown in FIG. 4. Each trench 122 hasopposing sidewalls lined with a dielectric layer 123, a shield electrode124 disposed between the sidewalls near the bottom the trench, adielectric layer 125 disposed over shield electrode 124, and a gateelectrode 126 disposed over the dielectric layer and between the trenchsidewalls. Each mesa 130 comprises a p-type well 134 disposed in theepitaxial semiconductor layer 104 adjacent to the top surface of layer104, a pair of source (n+ type) regions 136 disposed in p-well 134adjacent to two adjacent trenches 122 and the top surface of theepitaxial semiconductor layer 104, and an N-drift region 132 disposedbelow p-well 134. (A p-type well, a p-type region, and a p-doped regiondescribed herein may be referred to as “a well region of a firstconductivity type” or “a well region of a second conductivity type”,depending upon the context of the discussion or the context of theclaim.) A small trench is formed in the center of mesa 130 to allow thesource metal layer 110 to make electrical contact to the source regions136, and to the p-well 134 at a small region 135 of enhanced p+ doping.Electron current is conducted vertically through the device, from sourceregions 136, through an inverted region of the p-well 134 adjacent tothe gate oxide 123, further through drift region 132, and down to the N+substrate 102 and the drain contact, with the amount of current beingmodulated by the potential on the gate electrodes 126 in trenches 122under normal operating conditions. The shield electrodes 124 areelectrically coupled to the potential of the source metal layer 110 andsource regions 136, and shield the p-well from high electric fields.

When the potential on the gate electrode 126 is set to place the devicein an off state (e.g., typically a potential of around zero volts), asubstantial current can still flow during a breakdown condition wherethe drain potential is very high relative to the source potential. Inthe breakdown condition, high electric fields develop in a region ineach mesa 130, and this high electric field generates avalanche carriers(both holes and electrons). The voltage at which this breakdowncondition occurs is called the breakdown voltage. The breakdown voltageof the mesa may be raised by selecting the shield oxide thickness, thewidth of the mesa, and the doping of the N-drift region 132 to cause theN-drift region 132 to be normally depleted of electrons. This causes theelectric field during off-state conditions to be more uniformlydistributed along the centerline of the mesa (e.g., a square-shapedelectric field profile), thereby reducing the peak electric field (andthereby increasing the voltage at which avalanche carriers can begenerated). The condition whereby the N-drift region 132 is depleted ofelectrons is called the “charge-balanced condition.” The charge-balancedcondition can be generally achieved when the product of the mesa widthand the doping of the N-drift region 132 is in the range of 1×10¹¹ cm⁻²to 1×10¹³ cm⁻².

Ideally, one would like the breakdown voltage to be determined by thebreakdown process associated with mesa 130. However, various parasiticbreakdown mechanisms occur in various field termination regions of thedevice at lower voltages, and thereby set the overall breakdown voltageof the device to a lower value than that caused by the breakdown processin mesa 130. One such potential parasitic mechanism can occur at thethin portion of dielectric layer 123 in the outermost trench of a deviceregion 120 designed with a termination region of the prior art. Withouta mesa 130 next to it, this thin dielectric layer would be exposed tothe potential of the n-type epitaxial layer, which is coupled to thedrain potential, and a large electric field can develop across the thindielectric layer, which can cause a breakdown to occur at a relativelylow voltage.

One feature according to the present invention addresses this parasiticbreakdown mechanism by disposing an end trench 222 on either side of thearray of active trenches 122 of the device region 120. Trench 222 hasopposing sidewalls lined with a dielectric layer 223, a shield electrode124 disposed between the sidewalls near the bottom the trench, adielectric layer 125 disposed over shield electrode 124, and a gateelectrode 226 disposed over dielectric layer and between the trenchsidewalls. However, unlike the dielectric layer 123 of trench 122,dielectric layer 223 is thicker along the sidewall that faces the n-typeepitaxial layer than along the side wall that faces the trenches 122 ofdevice region 120, as measured along the depth of gate electrode 226.The thicker region is indicated by reference number 227 in the figure.The thicker dielectric reduces the electric field in the dielectriclayer, and thereby increases its breakdown voltage. Trench 222 may havethe same width as each of trenches 122, and gate electrode 226 may havea smaller width than gate electrode 126.

The above trenches 222, 122 and mesas 238, 230, and 130 are indicated inthe top plan view of FIG. 3 near the cross-section line indication forFIG. 4. A similar arrangement of trenches and mesas is present on theopposite side of device area 120, as indicated by these referencenumbers in the top plan view of FIG. 2. While the pair of trenches 222bound the array of trenches 122 and mesa 130 on either side of the array(e.g., the top and bottom of the array), they do not encircle the arrayor have portions that bound the right and left sides of the array. Thatis, there is no perpendicular termination trench at the ends of trenches122 and mesas 130. (It should be noted that trenches 122 and mesas 130continue to run under the gate runner.) Related to this, device 100 doesnot have a p-doped region disposed at the ends of trenches 122. Each ofthese features reduces the size of the field termination areas, andenables the active area to be increased and/or the die size to bedecreased. While the above configuration is for a device region 120 thatprovides a MOSFET device, it can also apply to other device types, suchas IGBT devices and rectifiers, particularly those devices in which theabove-described charge-balanced condition exists.

Referring back to FIG. 5, as another feature of the present invention,the broad mesa 238 to the left of end trench 222 may optionally have ap-type region 239 disposed at its surface, next to dielectric layer 223.P-type region 239 may be directly decoupled from any potential, and leftin a floating state, or may be electrically coupled to the source metallayer 110 and the source potential (e.g., it may be grounded). In eithercase, region 239 reduces the electric fields around the top right cornerof broad mesa 238, to eliminate this area as a source of parasiticbreakdown mechanism. When electrically coupled to the source potential,p-type region 239 further shields dielectric 223 from the drainpotential in area 227. P-type region 239 may be manufacturing during thesame process that manufactures p-wells 134.

As another feature of the present invention, the mesa 230 to the rightof end trench 222 may be configured as a p-n diode rather than a MOSFETtransistor. For this, it may comprise a p-well 134 and enhanced p+doping region 135, but no source regions 136. The p-n diode is biased inan off state during normal operations of the MOSFET transistor of deviceregion 120. Mesa 230 provides additional spacing distance between broadmesa 238 and the first active mesa 130 that serves to buffer thepotential in broad mesa 238 from the first active mesa 130. This enablesthe electrical characteristics of the first mesa 130 to be substantiallythe same as the interior mesas 130.

FIG. 6 shows a magnified cross section view of a portion of a variationof the exemplary semiconductor die of FIG. 1 according to the presentinvention. The features in the magnified cross section of FIG. 6 are thesame as those shown in the magnified cross section of FIG. 5, with theaddition of a perimeter trench 220, dielectric layer 221, and shieldelectrode 124. Trench 220 has opposing sidewalls lined with dielectriclayer 221, and a shield electrode 224 disposed between the sidewalls,preferably from the top of the epitaxial semiconductor layer to near thebottom of the trench. Shield electrode 224 is electrically coupled tothe source metal layer 110. Shield electrode 224 provides additionalshielding of the drain potential for end trench 222 and gate electrode226. A mesa 230′ is defined between trenches 220 and 222. P-doped region239 may be included in mesa 230′ between trenches 220 and 222, oromitted. Also, a p-doped region 234 that is disposed in mesa 230′ andthat extends from trench 222 to trench 220 may be used. Along withregion 234, a p-doped region 239′ may be included on the left side oftrench 220. A pair of trenches 220 bound the array of trenches 122, 222and mesas 130, 230, 230′ on either side of the array (e.g., the top andbottom of the array), but they do not encircle the array or haveportions that bound the right and left sides of the array. This featurereduces the size of the field termination areas, and enables the activearea to be increased and/or the die size to be decreased. While theabove configuration is for a device region 120 that provides a MOSFETdevice, it can also apply to other device types, such as IGBT devicesand rectifiers, particularly those devices in which the above-describedcharge-balanced condition exists.

FIG. 7 shows a cross section view of the aforementioned trenches andmesas in connection area 150 just adjacent to the device area 120, alongthe cut line 7-7 defined in FIG. 3. A thin amount of oxide layer 106 isdisposed over each of mesas 130 and 230, and dielectric layer 107 isdisposed over gate electrodes 126 and 226, as well as the underlyingoxide layer 106. The optional perimeter trench 220, shield electrode221, and dielectric layer 221 are shown in dotted outline. There is nochange to the configuration of p-doped region 239 with respect to itsneighboring elements with respect to the cross sections shown in FIGS. 4and 5.

FIG. 8 shows a cross section view of the aforementioned trenches andmesas in connection area 150 under the gate runner, along the cut line8-8 defined in FIG. 3. A thin amount of oxide layer 106 is disposed overeach of mesas 130 and 230. The tops of gate electrodes 126 and 226 areelectrically coupled together by a conductive riser 126R. Riser 126R iselectrically isolated from mesas 130, 230 by thin portions of oxide 106.In typical embodiments, riser 126R and gate electrodes 126, 226 areformed of the same material, such as polysilicon. In prior crosssections, the riser 126R is removed. The metal gate runner makes contactto riser 126R at locations over gate electrodes 126 and 226, which areseparated by islands of dielectric 107. The islands may be omitted. Thegate electrodes 126 and 226 terminate in the trenches at this point. Theoptional perimeter trench 220, shield electrode 221, and dielectriclayer 224 are shown in dotted outline. There is no change to theconfiguration of p-doped region 239 with respect to its neighboringelements with respect to the cross sections shown in FIGS. 4 and 5.

FIG. 9 shows a cross section view of the aforementioned trenches andmesas in connection area 150 between the gate runner and the shieldrunner, along the cut line 9-9 defined in FIG. 3. Only the shieldelectrodes 124 and 224 are present in trenches 122 and 222, with oxidelayer 106 covering them and the mesas 130 and 230.

FIG. 10 shows a cross section view of a trench 122 in connection area150 along a cut-line 10-10 defined in FIG. 3, with the cut-line 10-10being perpendicular to cut lines 4-4, 7-7, 8-8, and 9-9. Gate electrode126 and shield electrode 124 are disposed in the trench, with gateelectrode 126 having a riser 126R that makes electrical contact to thegate runner, and with shield electrode 124 having a riser portion 124Rthat makes electrical contact to the shield runner. Dielectric layer 125is disposed between shield electrode 124 and gate electrode 126 alongtheir facing horizontal dimensions, a dielectric layer 125S (which canbe referred to as a dielectric side layer) is disposed betweenelectrodes 124 and 126 along their facing side dimensions, and adielectric patch 125C (which can be referred to as a corner patch) ofdielectric is disposed between the outside corner of gate electrode 126and the inside corner of shield electrode 124. Shield electrode 124 hasan outside corner that is disposed adjacent to a patch 123C ofdielectric material, and a vertical side that is disposed adjacent to aside layer 123S of dielectric material.

Radius of curvature effects significantly increase the electric fieldsin the regions next to the outside corners of shield electrode and gateelectrode 126. The thickness of dielectric patch 123C is generallysufficient to prevent breakdown of the dielectric material. However,dielectric patch 125C and dielectric side layer 125S around gateelectrode 126 are relatively thin, and can be a source of breakdown forthe end trench 222 (shown in FIG. 8). The inclusion of optional shieldelectrode 224 and trench 220 shields dielectric patch 125C anddielectric side layer 125S from the drain potential coupled tosemiconductor layer 104, and thereby reduces the electric fields indielectric patch 125C and dielectric side layer 125S. Another possiblearea of breakdown due to radius of curvature effects, particularly forhigh voltage devices, is present in dielectric side layer 123S at theend of shield riser portion 124R, as indicated by point “A” in FIG. 10.This potential breakdown can be significantly mitigated by extending thetopside shield runner metal (which is a conductive trace) overdielectric side layer 123S and beyond the end of trench 122 by adistance L1. Distance L1 may be equal to or greater than the depth oftrench 122. For lower voltage device applications, the possibility ofbreakdown at point “A” is very low, and the topside shield runner metaldoes not extend over dielectric side layer 123S or beyond the end oftrench 122, as indicated by edge “B” in the figure. This configurationresults in a thinner shield runner, and a smaller die.

FIG. 11 shows a cross section view of a mesa 130 in connection area 150along a cut-line 11-11 defined in FIG. 3, with the cut-line 11-11 beingperpendicular to cut lines 4-4, 7-7, 8-8, and 9-9. The p-doped well 134and the riser 126R for the gate electrodes 126 are shown at the right ofthe figure. Typically, p-doped well 134 is electrically coupled to thepotential of the source and shield, but may be in a floating state forsome instances where the region is used in a field termination area.P-doped well 134 has an end that terminates at or under gate riser 126R(which is an electrical trace). For reference, the outlines of gateelectrode 126 and shield electrode 124 are shown in dashed lines. Thereis a possibility of breakdown occurring at the end of p-doped well 134due to radius of curvature effects. However, the gate electrodes 126 andshield electrodes 124 that are disposed on either side of p-doped well134 normally deplete the portion of n-doped mesa 130 that is adjacent tothe end of well 134, thereby significantly reducing the potential andelectric fields around the end of well 134. However, electric fields ofreduced amounts are still present around the ends of well 134, and canconcentrate at the end of well 134 in a radial manner (i.e., radius ofcurvature effect). However, with the configuration shown in FIG. 11, theend of well 134 is substantially shielded by gate riser 126R, andsubstantially reduces the radius of curvature effects at the region'send. Specifically, conductive riser 126R directs the electric fieldspresent in mesa 130 at the end of well 134 away from the end of well 134and towards itself, thereby reducing the radial concentration of theelectric field. This shielding would be lost if the end of well 134 wereto extend to the left side of the lower portion of conductive riser126R. This shielding effect is best obtained if the end of well 134 isspaced from the most distal side (e.g., left side) of the lower portionof conductive riser 126R by a distance L2, where L2 is equal to orgreater than the depth of well 134. In preferred implementations, L2 isequal to or greater than the depth of well 134 plus the separationdistance between well 134 and conductive riser 126R, where theseparation distance is equal to the thin portion of oxide layer 106 forthe configuration shown in the figure.

As mentioned above, the gate electrodes 126 and shield electrodes 124that are disposed on either side of p-doped well 134 normally depletethe portion of the n-doped mesa 130 that is adjacent to the end of well134, thereby significantly reducing the potential and electric fieldsaround the end of well 134. To achieve this benefit, the end of p-dopedregion should be spaced from the ends of the shield electrodes 124, orthe ends of the trenches 122, by at least a distance L3, as shown inFIG. 12. Distance L3 may be equal to the depth of trench 122, or may beequal to the difference between the depth of trench 122 and the depth ofwell 134. It is possible for well 134 to extend beyond gate riser 126R,as is shown in FIG. 13, and further possible for the end of well 134 tobe disposed under the shield runner (and field plate). If the shieldrunner is disposed near or over the end of well 134, it can provideshielding to mitigate the radius of curvature effects at the end of well134 in the same manner that gate riser 126R provided shielding, aspreviously described with reference to FIG. 11. However, for low andmoderate voltage applications, the shield runner need not be disposedover the end of p-doped well 134. While it is preferable that no otherp-doped regions are disposed between the end of well 134 and the ends ofthe adjacent trenches, a more lightly p-doped region may be disposedbetween the end of well 134 and the ends of the adjacent trenches. Themore lightly p-doped region has a lower dopant dose, as measured acrossa cross section of the mesa width, than that of well 134. Said anotherway, the more lightly p-doped region has a lower integrated changeresulting from the dopant, as measured across a cross section of themesa width, than that of well 134. With the above configurations, thereis no need for a termination trench running perpendicular to the ends oftrenches 122, as would be done in prior art configuration. All of theabove configurations of the end of p-doped well 134 may be applied totrench-shielded Schottky barrier diode devices, where the above spacingdistances are applied to the end of the Schottky metal, or if a p-dopedregion like region 239′ shown in FIG. 6 is used around the perimeter ofthe Schottky metal.

Referring back to FIG. 10, it may be seen that the shield runner metalmakes electrical contact with a top surface of the riser portion 124R ofshield electrode 124 at a level that is at or below the top surface ofepitaxial layer 104. This feature is also shown in FIG. 14, which is across section that is perpendicular to the cross section of FIG. 10. Asseen in FIG. 14, the contacts from the shield runner metal to the riserportions 124R are made through contact openings formed throughdielectric layer 107 and oxide layer 106. This configuration has theadvantages of reduced electrical contact resistance, and asimplification of the manufacturing process. In the conventionalmanufacturing processes, a polysilicon etch mask and etching step areused to define a polysilicon bus structure between the shield runnermetal and the shield electrodes 124, 224. However, the above simplifiedcontact structure can be defined by modifying an earlier mask that isused in the process, such as the mask used to define the contacts fromthe source metal to enhanced doping regions 135 and source regions 136shown in FIG. 5. Accordingly, the mask and etching step that areconventionally used to define the above-described polysilicon busstructure may be eliminated.

When making a high current capacity device, several instances of deviceregion 120 may be used rather than one large device region 120. Theinstances of device region 120 are electrically coupled in parallel, andthis configuration provides a low-resistance path to the centers of theshield electrodes 124 and the centers of the gate electrodes 126compared to the case where one large instance of device regions 120 isused. FIG. 15 shows a top schematic plan view of a semiconductor device200 disposed on a semiconductor die. Device 200 comprises a top deviceregion 120A disposed over a bottom device region 120B, a top connectionregion 150 (as previously described above) disposed above top deviceregion 120A, a bottom connection region 150 disposed below device region120B, a middle connection region 250 disposed between device regions120A and 120B. Device regions 120A and 120B are instances ofpreviously-described device region 120. There is a gate runner andshield runner in each connection region 150, and two gate runners andone shield runner in middle connection region 250. The gate runners areelectrically coupled to a gate pad 212 by a gate feed. Source metallayer 110 is disposed over device regions 120A and 120B, an electricallycoupled to the shield runners and two source pads 111. A plurality ofinterleaved trenches 122′ and mesas 130′ are disposed in thesemiconductor epitaxial layer, and within the device regions 120A, 120Band connection regions 150, 250, as illustrated by the dashed lines atthe right side of the figure. Only the first few trenches and mesas areshown for visual clarity in the figure, but the arrow symbols to theleft of the array schematically indicate that the array of interleavedtrenches and mesas extends to the left sides of the device regions 120A,120B and connection regions 150, 250. Trenches 122′ are substantiallythe same as trenches 122, except they run continuously through deviceregions 120A, 120B and connection regions 150, 250. Mesas 130′ aresubstantially the same as mesas 130, except they run continuouslythrough device regions 120A, 120B and connection regions 150, 250. Achannel stopper structure surrounds regions 120A, 120B, 150, and 250 atthe perimeter of the die, and is separated from regions 120A, 120B, 150,and 250 by a gap. This gap is the same as the gap shown an identified inFIG. 11.

FIG. 16 shows a cross sectional view of connection region 250 along thecut line 16-16 shown in FIG. 15. The cross section is taken along atrench 122′. The components are the same as the components describedabove with reference to FIG. 10, with the exceptions that a mirror setof a gate electrode 126, gate riser 126R, and gate runner appear at theleft side of the figure, that the dielectric patch 123C and dielectricside 123S are not present, and that trench 122′, dielectric layer 123′,shield electrode 124′, and shield riser 124R′ are mirrored over to theleft side and run continuously along the cross section from left toright. The gate runners make electrical contact the to gate risers 126Rof the gate electrodes 126, but are electrically insulated from theshield metal runner and the shield riser 124R′ and the shield electrode124′. The shield runner metal makes electrical contact to the shieldriser 124R′ and shield electrode 124′, and is electrically insulatedfrom the gate runners, gate risers 124R, and gate electrodes 124. Theabove trench construction eliminates the imbalances in the electricfields and potentials that occur in the mesa regions of device 100 thatare next to the trench discontinuities in connection regions 150, andthus eliminate the corresponding localized charge imbalances. Thisconstruction is a departure from prior art constructions, which wouldinclude elaborate field termination structures in the middle ofconnection region 250.

FIG. 17 shows a cross sectional view of connection region 250 along thecut line 17-17 shown in FIG. 15. The cross section is taken along a mesa130′. The components are the same as the components described above withreference to FIG. 10, with the exception that a mirror set of a p-dopedwell 134, a gate electrode 126, a gate riser 126R, and a gate runnerappear at the left side of the figure. Outlines of the positions ofshield electrode 124′ and gate electrodes 126 are shown in dashed lines.Similar to device 100, each p-doped well 134 is typically electricallycoupled to the potential of the source and shield, but may be in afloating state for some instances where the region is used in a fieldtermination area. Each p-doped well 134 has an end that preferablyterminates at or under a respective gate conductive riser 126R (which isan electrical trace). There is a possibility of breakdown occurring atthe end of each p-doped well 134 due to radius of curvature effects.However, the gate electrodes 126 and shield electrodes 124 that aredisposed on either side of the p-doped well 134 normally deplete theportion of each n-doped mesa 130 that is adjacent to the end of the well134, thereby significantly reducing the potential and electric fieldsaround the end of the well 134. However, electric fields of reducedamounts are still present around the ends of the well 134, and canconcentrate at the end of the well 134 in a radial manner (i.e., radiusof curvature effect). However, with the configuration shown in FIG. 17,the ends of the wells 134 are substantially shielded by the gateconductive risers 126R, and the configuration substantially reduces theradius of curvature effects at the regions' ends (as previouslydescribed above for device 100 with reference to FIG. 11). Thisshielding would be lost if the end of a well 134 were to extend beyondthe distal side of the lower portion of the conductive riser 126Rdisposed above it. This shielding effect is best obtained if the end ofthe well 134 does not extend beyond the distal side of the lower portionof the conductive riser 126R, and is further spaced from the most distalside of the lower portion of conductive riser 126R by a distance L2,where L2 is equal to or greater than the depth of well 134. In preferredimplementations, L2 is equal to or greater than the depth of well 134plus the separation distance between well 134 and conductive riser 126R,where the separation distance is equal to the thin portion of oxidelayer 106 for the configuration shown in the figure.

FIG. 18 shows a cross sectional view of a variation 250′ of connectionregion 250 that includes an electrically floating p-doped well 134Cdisposed in epi layer 104, and under the shield runner metal. (Thep-doped region is made floating by not making a direct electricallyconnection between it and a conductive layer that is adapted to receivean electrical potential from an external circuit.) The floating p-dopedwell 134C acts as a buffer shield between epi layer 104 and the portionof the oxide layer 106 above it. The portion of oxide layer 106 betweenthe shield runner metal and epi layer 104 can experience high electricfields since the shield runner metal is usually at ground potential andthe underlying portion of epi layer 104 is usually at the drainpotential. To reduce radius of curvature effects at the ends of p-dopedwell 134C, the ends may be disposed near or under the gate risers 126R.

FIG. 19 shows a cross sectional view of another variation 250″ ofconnection region 250 that includes a continuous p-doped well 134′ inplace of the two wells 134 shown in FIG. 17. Continuous p-doped well134′ extends from device region 120A to device region 120B, and throughconnection region 250″, and is electrically coupled to the source metallayer 110 (and, in turn, to the shield runner metal). There is no radiusof curvature effect associated with continuous p-doped well 134′ becausewell 134′ does not have a side edge or corner. Continuous p-doped well134′ also acts as a buffer shield between epi layer 104 and the oxidelayer 106 above it. As indicated above, the portion of oxide layer 106between the shield runner metal and epi layer 104 can experience highelectric fields since the shield runner metal is usually at groundpotential and the underlying portion of epi layer 104 is usually at thedrain potential.

In all of the embodiments illustrating connection regions 150, 250,250′, and 250″, it may be appreciated that each connection region has aconfiguration of one more material bodies with the adjacent portions ofmesas 130, 130′ which produces an inactive device. A material body maycomprise a doped region, a dielectric layer, a conductive layer, etc. Incontrast, each device region 120, 120A, 120B has a configuration of onemore material bodies with portions of the mesas 130, 130′ which producesan active device.

Another embodiment is now described and illustrated with reference tosemiconductor device 300 illustrated in FIG. 20. Semiconductor device300 has substantially the same floor plan (top plan view) assemiconductor device 100 shown in FIGS. 1-3. FIG. 20 is a magnified viewof a portion along the left side of the die of semiconductor device 300,similar to the magnified view of the left side portion of device 100shown in FIG. 3. Semiconductor device 300 comprises substantially thesame elements as device 100 arranged in substantially the same way, andfurther comprises a perimeter trench 320 that encircles the array oftrenches 122,222 and mesas 130,230 previously described above. FIGS. 21and 22 shows cross sections of perimeter trench 320 and the array oftrenches 122,222 and mesas 130,230 along the bottom of the array, andalong the cut lines 21-21 and 22-22 shown in FIG. 20. Perimeter trench320 comprises a dielectric layer 321 lining its opposing side walls, anda conductive electrode 324 disposed in the trench. Conductive electrode324 may be electrically coupled to a conductive layer, such as theshield runner, to receive a ground potential, or may be decoupled fromany conductive layer bearing a potential, thereby being at a floatingpotential. Perimeter shield 320 is spaced from trench 222 by a distancethat is on the order of the spacing between adjacent trenches 122. A gapregion 330 is disposed between perimeter shield 320 and trench 222. Noelectrical potentials are coupled to the top of gap region 330 by anyconductive layer, and the potential in gap region 330 is floating. Whenthe perimeter trench electrode 324 is at a floating potential, thepotentials on it and the floating gap region 330 can float to setequalizing potentials with respect to the drain potential, and canthereby reduce sensitivity to charge imbalances in gap region 330. As aresult, achieving the charge balance condition in gap region 330 becomeseasier than if these gap regions 330 were fixed at source potential byconventional grounded p-wells. Substantially the same benefits areachieved when the perimeter trench electrode 324 is coupled to a groundpotential. The width of gap region 330 may be equal to or less than 1.25times the width of mesa 130, and the widths of gap region 330 along thevarious sides of perimeter trench 320 may be different. For example, thewidth of gap region 330 along the left and right vertical sides ofperimeter trench 320 (and the main array of trenches 122 and mesas 130)may be smaller than the width of gap region 330 along the top and bottomhorizontal sides of perimeter trench 320 (and the main array).

FIGS. 23 and 24 are cross sections showing perimeter trench 320 alongthe ends of the primary trenches 122 and mesas 130, and along the cutlines 23-23 and 24-24 shown in FIG. 20. The cross-sections of FIGS. 23and 24 are substantially the same as the cross sections of FIGS. 10 and11 for device 100, plus the addition of perimeter trench 320 and gapregion 330. Elements 102-107, 120, 122, 123, 123C, 123S, 124, 124R, 125,125C, 125S, 126, 126R, 134, 150, the shield runner, the gate runner, andchannel stopper have their same relative relationships with respect toone another. As indicated above, one possible area of breakdown due toradius of curvature effects, particularly for high voltage devices, ispresent in dielectric side layer 123S at the end of shield riser portion124R, as indicated by point “A” in FIG. 23 (the same as was for FIG.10). As previously described, this possible area of breakdown can besignificantly mitigated by extending the topside shield runner metal(which is a conductive trace) over dielectric side layer 123S and beyondthe end of trench 122 by a distance L1. Distance L1 may be equal to orgreater than the depth of trench 122. Perimeter trench 320 alsomitigates the possible area of breakdown by moving electric fields awayfrom point A. As indicated above, when the perimeter trench electrode324 is at a floating potential, the potentials on it and the floatinggap region 330 can float to set equalizing potentials with respect tothe drain potential, and can thereby reduce sensitivity to chargeimbalances in gap region 330. As a result, achieving the charge balancecondition in gap region 330 becomes easier than if these gap regions 330were fixed at source potential by the conventional grounded p-dopedwells. Substantially the same benefits are achieved when the perimetertrench electrode 324 is coupled to a ground potential, which may be doneby a contact via 325 of conductive material disposed between shieldelectrode 324 and the shield runner metal, with the conductive contactvia 325 being electrically coupled to both the shield runner and shieldelectrode 324.

The same above benefits can be substantially achieved with the use of afloating p-doped well 334 in floating gap region 330. This embodiment isillustrated by FIGS. 25-28, which are the same cross-sections as FIGS.21-24, with the exception of the addition of the floating p-doped well334. No ground potential voltage is coupled to well 334. FIG. 29 showsfloating p-doped well 334 in the portion of gap region 330 that isdisposed between trench 222 and the perimeter trench 320. Well 334extends left to be adjacent to perimeter trench 320. While well 334 hasbeen shown as a continuous stripe that is disposed adjacent to perimetertrench 320, it may be appreciated that well 334 may be segmented (havinggaps in the continuous stripe). The ends of any segmented region of well334 may be disposed under a shield runner and other conductive traces tominimizing radius of curvature effects.

When using perimeter trench 320, either with a grounded or floatingelectrode 324, there can be a charge imbalance at the corner turns ofperimeter trench 320. This is because gap region 330 sees two sides ofperimeter trench 320 instead of one, as shown in the magnified top planview of FIG. 30. The electrode 324 of the perimeter trench tries todeplete more charge than that present in the corner area of gap region330. This charge imbalance can be addressed by shortening the length oftrench 222 that is adjacent to the horizontal leg of perimeter trench320. This presented as device 400, which is the same as device 300except for the shorting of the trench. The shortening of trench 222reduces the charge imaging effect of trench 222 on the corner of gapregion 330, and thereby compensates for the over imaging of electrode324 of the perimeter trench. FIG. 31 shows a cross section of shortenedtrench 222, along with an outline of the un-shortened length forcomparison. The end of trench 222 is spaced further from perimetertrench 320 than the ends of trenches 122. The p-well 334 of device 300may be added to device 400, with any of the above-describedconfigurations of device 400. The p-well 334 for device 400 may be at afloating potential or at a fixed potential (e.g., ground potential).

FIG. 32 shows a top view of another exemplary semiconductor device 500that incorporates several features according to the present invention.Device 500 comprises an active device region 120, as previouslydescribed above with reference to device 100, located in the middle ofthe die, along with source metal layer 110, source pads 111, gate pad112, and connection regions 150 as previously described above. Withoutloss of generality, device region 120 may implement a vertical,trench-shielded power MOSFET device. As described and shown below ingreater detail, the exemplary MOSFET device comprises an array oftrenches interleaved with an array of mesas, insulated shield electrodesdisposed in bottoms of the trenches, insulated gate electrodes disposedin the trenches over the shield electrodes, source regions disposed inthe mesas, source electrodes disposed on the source regions, and a drainelectrode provided at the backside of the semiconductor device. Eachsource pad 111 and gate pad 112 is adapted to receive an externalconnection, such as a wire bond or solder bump that provides a sourcepotential, and may have dimensions of 150 microns or more on each side.

A channel stopper is disposed at or near the periphery of the die, andis spaced from the shield runners and the top portion of device region120 by a gap. The channel stopper is conventional, and may comprise anisolated ring of metal that overlays and makes contact to a strip ofdoped semiconductor region that forms a ring around the periphery of thedie. Of significant note, like device 100, device 500 does not comprisethe conventional field termination structures that would normally appearin this gap. Cross-sections of device 500 in active area 120, connectionareas 150, and left and right inactive areas, and upper inactive area,may be substantially the same as those shown for device 100 and itsvarious variations in FIGS. 5-14, previously described above.

As a difference with device 100, device 500 comprises a plurality oftrenches 522, 522′, 522″ and mesas 530, 530′, 530″ disposed under gatepad 112, the left and right gate runners, and a bottom shield runner, asillustrated in the figure. Typically, trenches 522 and mesas 530 do notextend into device region 120 to any significant degree (the bottom edgeof device region 120 may cover one of trenches 522). Trenches 522, 522′,522″ may have the same constructions as illustrated above for trenches122, 220, 222, or modified constructions thereof, and mesas 530, 530′,530″ may be un-doped, or doped to have well regions therein similar tomesas 130. As described below in greater detail, each of trenches 522,522′, 522″ may have shield electrodes that are electrically floating orelectrically coupled to source pad 111 (e.g., grounded). Each oftrenches 522, 522′, 522″ may include a gate electrode, or may exclude agate electrode. Similarly, each of mesas 530 may be electricallyfloating or electrically coupled to source pad 111 (e.g., grounded).Source metal layer 110 may comprise source metal extensions 110 a and110 b (which can be referred to as source extensions or as sourcerunners) that extend downward on either side of gate pad 112, and may beselectively coupled to various ones of trenches 522, 522′, 522″ andmesas 530, 530′, 530″ to provide a ground potential thereto. Trenches522, 522′, 522″ and mesas 530, 530′, 530″ aid in shaping the electricfields at the bottom edge of device region 120, and hence aid incontrolling the breakdown voltage characteristics of device 500.Trenches 522, 522′, 522″ and mesas 530, 530′, 530″ also aid in shieldinggate pad 112 from the voltage applied to the drain electrode at thebottom surface of device 500, thereby reducing the capacitance betweenthe device's gate and drain electrodes. The amount of shielding can beselected by the construction of trenches 522, 522′, 522″ and mesas 530,530′, 530″, and varying the amount of these trenches and mesas that arein floating or grounded states, as described below in greater detail.

FIG. 33 is a cross section view of a portion of the device 500 takenalong the cross section line 33-33 shown in FIG. 32. The cross sectiongoes under gate pad 112 and the shield runner disposed adjacent to gatepad 112. Device 500 comprises a N+ doped semiconductor substrate 102,one or more epitaxially grown semiconductor n-type layers 104(“epitaxial semiconductor layer”) disposed on semiconductor substrate102, an oxide layer 106 disposed over epitaxial semiconductor layer 104in the inactive and first field termination regions, and a dielectriclayer 107 disposed over the oxide layer 106. Gate pad 112 and the shieldrunner are disposed over the dielectric layer. As an artifact of anexemplary manufacturing process and not a limitation on the constructionof device 500, oxide layer 106 ends at a point “D” under gate pad 112.This point roughly corresponds to the lower ends of source extensions110 a and 110 b, and is noted in FIG. 32.

Device 500 further comprises a plurality of trenches 522, 522′, 522″disposed in the epitaxial semiconductor layer, and a plurality of mesas530, 530′, 530″ of semiconductor material interleaved between trenches522, 522′, 522″ arranged as shown in FIG. 33. Trench 522 has aconstruction similar to that of trench 122 (shown in FIGS. 6-7), havingopposing sidewalls lined with a dielectric layer 523, a shield electrode524 disposed between the sidewalls near the bottom the trench, adielectric layer 525 disposed over shield electrode 524, and a secondelectrode 526 (which may comprise a gate electrode and can be referredto as a gate electrode) disposed over the dielectric layer 525 andbetween the trench sidewalls. Trench 522′ has a construction similar tothat of trench 222 (shown in FIGS. 6-7), having opposing sidewalls linedwith a dielectric layer 523′, a shield electrode 524 disposed betweenthe sidewalls near the bottom the trench, a dielectric layer 525disposed over shield electrode 524, and a gate electrode 526′ disposedover dielectric layer and between the trench sidewalls. However, unlikethe dielectric layer 523 of trench 522, dielectric layer 523′ is thickeralong the sidewall that faces the termination edge than along the sidewall that faces the trenches 522, as measured along the depth of gateelectrode 526′. The thicker region is indicated by reference number 527in the figure. The thicker dielectric region 527 reduces the electricfield in the dielectric layer, and thereby increases its breakdownvoltage. Trench 522′ may have the same width as each of trenches 522,and gate electrode 526′ may have a smaller width than gate electrode 526of trench 522. Trench 522″ has a construction similar to that of trench220 (shown in FIGS. 6-7), having opposing sidewalls lined withdielectric layer 523″, and a shield electrode 524″ disposed between thesidewalls, preferably from the top of the epitaxial semiconductor layer104 to near the bottom of the trench. Each of shield electrodes 524,524″ may be electrically floating or electrically coupled to the sourcemetal layer 110. Each of gate electrode 526, 526′ may be electricallyfloating, or electrically coupled to gate pad 112, or electricallycoupled to source metal layer 110. Trench 522″ provides additionalshielding of the drain potential for trenches 522 and 522′.

Each mesa 530 is disposed between two adjacent trenches 522, 522′, andmay comprise a p-type well 534 disposed in the epitaxial semiconductorlayer 104 adjacent to the top surface of layer 104. Mesa 530′ isdisposed between trenches 522′ and 522″. A p-doped region 539 may beincluded in mesa 530′, or omitted. Also, a p-doped well 534 may bedisposed in mesa 530′, with the well extending from trench 522′ totrench 522″. Each mesa 530″ is disposed between two adjacent trenches522″, 522′. A p-doped well 534 may be included in each mesa 530″, oromitted as shown in FIG. 33. As described below, each of wells 534 maybe coupled to the potential of the source layer 110, or may beelectrically isolated and left in an electrically floating state. Thewidths of mesas 530, 530′, and 530″ may be selected such that the n-typeregions within the mesas are depleted of electrons (e.g., carriers) byelectrodes 524, 524′, and 524″, under the above-describedcharge-balanced condition. As described above, the charge-balancedcondition can be generally achieved when the product of the mesa widthand the doping of the N-drift region of the mesa is in the range of1×10¹¹ cm⁻² to 1×10¹³ cm⁻². As noted above, mesas 130 of the activeregion are generally designed to provide this condition. Typically, thecharge-balanced condition for mesas 530, 530′, and 530″ can be achievedby selecting the widths of 530, 530′, and 530″ to be equal to or lessthan 1.25 times the width of mesa 130 of the device region 120. If mesa530″ is configured to be in a charge-balanced condition and does nothave a p-doped region 539, it will be in an electrically floating state.

As indicated above, the trenches 522, 522′, 522″, gate electrode 526,526′, and mesas 530, 530′, and 530″ may be individually configured to bein an electrically floating state, or electrically coupled to apotential. Exemplary configurations thereof are illustrated by thesequence of cross sections that are described next.

FIG. 34 shows a cross section view of the aforementioned trenches andmesas along the cut line 34-34 defined in FIG. 32. In this area, sourceextension 110 a overlies trenches 522, 522′ and mesas 530, and a viathrough insulating layers 106-107 is formed to the center of each mesa530 to allow the source extension 110 a to make electrical contact toeach p-well 534 at a small region 535 of enhanced p-type doping.Portions of dielectric layer 107 cover the tops of trenches 522. Thisconfiguration electrically couples p-wells 534 and the tops of mesas 530to the potential of source metal layer 110 (e.g., ground). If a mesa 530and its well 534 are intended to be configured in a floating state, thenthe aforementioned via is not formed, and the cross section looks likethat shown in FIG. 33 except that the gate runner and source extension110 a replace the gate pad 112. The aforementioned vias and regions 535of enhanced p-type doping may be made with the same manufacturingprocess that is used to form p+ regions 135 and the vias of source metal110 to wells 134 in the device region, as illustrated in FIG. 6 anddescribed above. If this exemplary process is used in the area of thecross section shown in FIG. 34, source regions 636 similar to sourceregions 136 (shown in FIG. 6) may be disposed in the rightmost wellregions 534, as an artifact of the exemplary manufacturing process. Thesource regions 636 do not interfere with the electrical contact betweensource extension 110 a and wells 534, and they create small activedevices under source extensions 110 a and 110 b. If desired, the widthsof the source extensions can be widened to increasing the active area ofthe device by extending it on one or both sides of gate pad 112. Theleftmost well 534 does not have source regions 636, and is configured asa reverse-biased p-n diode. If desired, the masks of the exemplaryprocess may be readily modified to omit source regions 636 under sourceextension 110 a and 110 b. If gate electrodes 526 and 526′ are to becoupled to the source metal layer 110 (e.g., “grounded”), the photomaskmay be readily modified to remove the portions of layers 106 and 107located over the gate electrodes so that source runners 110 a and 110 bmay make electrically contact to the gate electrodes.

FIG. 35 shows a cross section view of trenches 522, 522′, 522″ and mesas530, 530′, and 530″ along the cut line 35-35 defined in FIG. 32. In thisarea, the tops of gate electrodes 526 and 526′ are electrically coupledtogether by conductive riser 126R, which was previously described abovewith reference to device 100. Riser 126R is electrically isolated frommesas 530 by oxide layers 106′ that are disposed over the tops of mesas530. In turn, riser 126R is electrically coupled to the gate runner andgate pad 112, as shown in FIGS. 35 and 32, and as described above withreference to device 100. In typical embodiments, riser 126R and gateelectrodes 526, 526′ are formed of the same material, such aspolysilicon. In some implementations, as described above, the islands ofdielectric 107 over trenches 522 may be omitted. If a gate 526 or 526′is intended to be configured in a floating state or ground to sourcemetal 110, then the aforementioned electrical connection with gate riser126R is not made, and the cross section looks like that shown in FIG. 33except that the gate runner replaces the gate pad 112. The photoresistmasks used to define riser 126R may be readily modified so as to notmake the connection.

FIG. 36 shows a cross section view of trenches 522, 522′, 522″ and mesas530, 530′, and 530″ along the cut line 36-36 defined in FIG. 32. In thisarea, a shield runner overlies trenches 522, 522′, 522″ and mesas 530,530′, and 530″. As seen in the figure, contacts from the shield runnerto shield electrodes 524 and 524″ of trenches 522, 522′, 522″ are madethrough vias (e.g., contact openings) formed through dielectric layer107 and oxide layer 106. This configuration electrically couples theshield electrodes 524 and 524″ to the source metal layer 110 by way ofthe shield runner, and to the potential received at source pads 111(e.g., ground). Each of shield electrodes 524 and 524″ may be placed inan electrically floating state by not forming a via between the shieldrunner and the shield contact. The photomask used to define the vias maybe readily modified to omit vias where desired. It should be noticedthat wells 534 and doped region 539 do not appear in the cross sectionsince these regions have been terminated before reaching the mid pointof the shield runner, as per the configurations for device 100 shown inFIGS. 11-13 and previously described above.

Floating trenches 522, 522′, 522″ and floating mesas 530, 530′, and 530″create a floating potential shield between gate pad 112 and the drainelectrode at the backside of substrate 102, which reduces thecapacitance between gate pad 112 and the drain electrode by a capacitivedividing action. In this configuration, the floating trenches andfloating mesas do not significantly impact the capacitance between gatepad 112 and the source pad 111. On the other hand, using “grounded”trenches 522, 522′, 522″ and “grounded” mesas 530, 530′, and 530″reduces the capacitance between gate pad 112 and the drain electrode toa significantly greater degree since these structures provide a fixedpotential shield between gate pad 112 and the drain electrode, withsubstantially no capacitive dividing action occurring. However, thisconfiguration significantly increases the capacitance between the gatepad 112 and the source pad 111. Different circuit applications generallybenefit from different ratios of the above-noted capacitances, and thusthe blend of floating trenches/mesas and “grounded” trenches/mesas maybe adjusted to provide a desired ratio, while at the same timepreventing avalanche breakdown from occurring in the regions under thegate pad 112. Typically, the grounded trenches/mesas are disposedadjacent to device area 120 and the floating trenches/mesas are disposedat the outer edge of gate pad 112 since the electrical structure of thegrounded trenches/mesas is more similar to that of trenches and mesas ofdevice area 120 than the structure of the floating trenches/mesas.

For areas where the mesas have wells 534 coupled to source metal layer110 (e.g., “grounded”), and where trenches 522 are configured liketrenches 122, the breakdown characteristics are substantially the sameas those in device area 120. For areas where the mesas have wells 534coupled to source metal layer 110 (e.g., “grounded”), and where trenches522″ are coupled to source metal layer 110, the spacing distance betweenthe trenches may have to be adjusted (e.g., narrowed) to provide a goodcharge-balanced condition (e.g., depletion of electrons) in the mesas.This adjustment makes the breakdown voltage characteristicssubstantially the same or better than those in device area 120. Suchadjustment may be done by one of ordinary skill in the art usingcomputer simulation or parametric test structures. For areas where themesas are floating, with or without wells 534, there is less risk ofbreakdown. However, the potential distributions in the floating areascan affect adjacent areas that use grounded trenches and mesas, and thespacing distance between the floating trenches and mesas should beadjusted (e.g., narrowed) to provide a good charge-balanced conditionfor the floating mesas so as to shape the potential distributions in theadjacent areas.

FIGS. 37-39 are various cross sections of another exemplarysemiconductor device 600 that incorporates several features according tothe present invention. Device 600 has the same top plan view as shown inFIG. 32 for device 500, and is substantially the same as device 500except for the incorporation of some additional features. Accordingly,the above description of device 500 applies to device 600 to the extentthat it does not conflict with the following description of device 600.FIG. 37 shows a cross-section of device 600 taken along the line 33-33shown in FIG. 32. As a first additional feature, device 600 comprisesthe same elements of device 500 plus a plurality of p-doped conductiveregions 634 disposed in mesas 530″ at the top surface of epitaxial layer104. Regions 634 may comprise wells that are deeper than wells 534, butwhich have doping levels that are comparable to that of wells 534 (e.g.,within 0.5 times to 2 times the doping of a well 534) to achieve a goodcharge-balanced condition. As a result of an exemplary manufacturingprocess for making regions 634, a first p-type tail region 639A isformed to the left of the leftmost trench 522″ and leftmost region 634,and a second p-type tail region 639B is formed to the right of therightmost trench 522″ and rightmost region 634. Second tail region 639Bmerges with region 539, to form a continuous p-type well in mesa 530′.As shown below, the well in mesa 530′ is electrically coupled to sourcemetal layer 110 (e.g., “grounded”) in another cross section of thedevice. Regions 634 and trenches 522, 522′, 522″ are also coupled tosource metal layer 110 in another cross section of the device. Firsttail region 639A is placed in an electrically floating state. Gateelectrodes 526 and 526′ are coupled to gate pad 112 in the same mannerthat gate electrodes 526 and 526′ in device 500 are coupled to gate pad112 via gate riser 126R, as shown in FIG. 35.

FIG. 38 shows a cross-section of device 600 taken along the line 34-34shown in FIG. 32 where source extension 110 a overlies trenches 522,522′ and mesas 530. This cross section is the same as that for device500 except for the addition of regions 634, 639A, and 639B, and theaddition of an electrical contact between source extension 110 a and thewell formed by the merger of regions 639B and 539. This electricalcontact comprises a small region 535 of enhanced p-type doping in themerged regions and a small via formed through insulating layers 106-107and to the p-type region 535 at the centerline of mesa 530′, whichallows the source extension 110 a to make electrical contact to each ofp-type regions 539 and 639B.

FIG. 39 shows a cross section of device 600 taken along the line 36-36shown in FIG. 32 where the shield runner overlies trenches 522, 522′,522″ and mesas 530, 530′, 530″. This cross section is the same as thatfor device 500 except some of the islands of dielectric layers 106 and107 are removed so that the shield runner can make electrical contact top-type regions 634 at the tops of mesas 530″. If the mesa widths andtrench widths are large enough, then contacts to the trenches and mesascan be made through separate vias (e.g., through individual vias ratherthan a single via).

With the above construction of device 600, the trenches and mesasdisposed below gate pad 112 are electrically coupled to source layer110, thereby shielding the entire area of gate pad 112 from the drainelectrode at the back surface of the die. This significantly reduces thegate-to-drain capacitance of the device, but does increase thegate-to-source capacitance.

While the above embodiments have been illustrated with n-type epi layersand p-type doped well regions, it may be appreciated that the inventionsand embodiments may be practiced with p-type epi layers and n-type dopedwell regions. In other words, the inventions and embodiments may bepracticed with the doping polarities of the layers and regions reversed.

While the various embodiments of the inventions are mostly described inthe context of N-channel shielded gate MOSFET, these embodiments may beimplemented in a variety of other types of devices, such as, P-channelMOSFET (i.e., a transistor similar in structure to the MOSFETs describedabove except that the conductivity type of all silicon regions arereversed); N-channel shielded gate IGBT (i.e., a transistor similar instructure to the MOSFETs described above except that a P-type substrateis used instead of the N-type substrate); P-channel shielded gate IGBT(i.e., a transistor similar in structure to the MOSFETs described abovebut with silicon regions of opposite conductivity except the substrateis kept N-type); shielded gate synchronous rectifiers (i.e., integratedshielded gate MOSFET and Schottky); TMBS rectifiers, and superjunctionvariations of the above devices (i.e., devices with columns ofalternating conductivity type silicon).

Any recitation of “a”, “an”, and “the” is intended to mean one or moreunless specifically indicated to the contrary.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, it being recognized that variousmodifications are possible within the scope of the invention claimed.

Moreover, one or more features of one or more embodiments of theinventions may be combined with one or more features of otherembodiments of the invention without departing from the scope of theinvention.

While the present inventions have been particularly described withrespect to the illustrated embodiments, it will be appreciated thatvarious alterations, modifications, adaptations, and equivalentarrangements may be made based on the present disclosure, and areintended to be within the scope of the invention and the appendedclaims.

The invention claimed is:
 1. A semiconductor device comprising: a firsttrench extending in a semiconductor region, the first trench having ashield electrode and a gate electrode vertically stacked therein, theshield electrode being insulated from the gate electrode; a source padconfigured to receive a first external connection and electricallycoupled to the shield electrode of the first trench; a gate pad disposedabove the semiconductor region and configured to receive a secondexternal connection; an electrical trace electrically coupled to thegate pad or to the source pad; a second trench extending in thesemiconductor region and disposed directly under at least one of thegate pad and the electrical trace, the second trench having a firstelectrode disposed therein, the second trench being insulated from theelectrical trace by a dielectric when directly under the electricaltrace; a first mesa of semiconductor material adjacent the first trench;and a second mesa of the semiconductor material adjacent the secondtrench.
 2. The semiconductor device of claim 1, further comprising: awell region of a first conductivity type disposed in the semiconductorregion, the well region being disposed in the second mesa, the wellregion being electrically floating or is electrically coupled to thesource pad.
 3. The semiconductor device of claim 1, further comprising:a well region of a first conductivity type disposed in the semiconductorregion, the well region being disposed in the second mesa, at least oneof the well region and the first electrode of the second trench beingelectrically coupled to the source pad.
 4. The semiconductor device ofclaim 1, wherein the gate electrode is a first gate electrode, thesecond trench includes a second gate electrode vertically stacked overand insulated from the first electrode, the second gate electrode iselectrically coupled to the gate pad.
 5. The semiconductor device ofclaim 4, further comprising: a well region of a first conductivity typedisposed in the semiconductor region, the well region being disposed inthe second mesa, the well region being electrically coupled to thesource pad or is electrically floating.
 6. The semiconductor device ofclaim 1, further comprising: a third trench extending in a semiconductorregion and disposed under the at least one of the gate pad and theelectrical trace, the third trench including a gate electrode verticallystacked over and electrically insulated from a shield electrode, thegate electrode being electrically coupled to the gate pad.
 7. Thesemiconductor device of claim 6, further comprising: a well region of afirst conductivity type disposed in the semiconductor region, the wellregion being disposed in the second mesa, the well region beingelectrically coupled to the source pad or electrically floating.
 8. Thesemiconductor device of claim 1, wherein the first electrode iselectrically coupled to the source pad, the semiconductor device furthercomprising: a third trench extending in a semiconductor region anddisposed under the at least one of the gate pad and the electricaltrace, the third trench including a second electrode that iselectrically floating.
 9. The semiconductor device of claim 8, furthercomprising: a well region of a first conductivity type disposed in thesemiconductor region, the well region being disposed in the second mesa,the well region being electrically coupled to the source pad orelectrically floating.
 10. The semiconductor device of claim 1, whereinthe second mesa has a width that is equal to or less than 1.25 times awidth of the first mesa.
 11. The semiconductor device of claim 10,wherein the first electrode is electrically coupled to the source pad.12. The semiconductor device of claim 1, wherein the electrical trace iselectrically coupled to the first trench.
 13. The semiconductor deviceof claim 1, wherein the first electrode is electrically coupled to thesource pad, the second mesa has a width that is equal to or less than1.25 times a width of the first mesa.
 14. The semiconductor device ofclaim 1, wherein the first electrode is electrically floating, thesecond mesa has a width that is equal to or less than 1.25 times a widthof the first mesa.
 15. The semiconductor device of claim 1, furthercomprising: a shield runner made of a metal and disposed above thesemiconductor region and having a portion contacting a top surface ofthe first electrode, the top surface of the first electrode being at orbelow a top surface of the semiconductor region.
 16. The semiconductordevice of claim 15, further comprising: a third trench including afloating electrode or including an electrode electrically coupled to thesource pad.
 17. A semiconductor device comprising: a first trenchextending in a semiconductor region, the first trench having a shieldelectrode and a gate electrode vertically stacked therein, the shieldelectrode being insulated from the gate electrode; a source padconfigured to receive a first external connection and electricallycoupled to the shield electrode of the first trench; a gate pad disposedabove the semiconductor region and configured to receive a secondexternal connection; an electrical trace electrically coupled to thegate; and a second trench extending in a semiconductor region anddisposed directly under at least one of the gate pad and the electricaltrace, the second trench having a first electrode disposed therein, thesecond trench being insulated from the electrical trace by a dielectricwhen directly under the electrical trace.
 18. The semiconductor deviceof claim 17, further comprising: a well region of a first conductivitytype disposed in the semiconductor region and adjacent the secondtrench, the well region being electrically floating or is electricallycoupled to the source pad.
 19. The semiconductor device of claim 17,further comprising: a well region of a first conductivity type disposedin the semiconductor region and adjacent the second trench, at least oneof the well region and the first electrode of the second trench beingelectrically coupled to the source pad.
 20. The semiconductor device ofclaim 17, wherein the first electrode is electrically coupled to thesource pad or electrically floating.
 21. The semiconductor device ofclaim 17, further comprising: a mesa adjacent to the second trench, themesa being a floating p-type mesa region.
 22. The semiconductor deviceof claim 21, wherein the mesa is a first mesa, the semiconductor devicefurther comprising: a second mesa adjacent the first trench, the firstmesa having a width that is equal to or less than 1.25 times a width ofthe second mesa.